Power converters including one or more transformers are typically used for converting a signal from a direct current (DC) source, such as photo-voltaic (PV) panels, to an alternating current (AC) signal. One reason power might need to be converted from DC to AC is to allow a DC power source to supply power for transmission on an AC electrical grid (referred to herein as “the grid”). Another reason power might need to be converted from DC to AC is to allow a DC power source to power an AC load.
Using transformers in power conversion—either line frequency or high (switching) frequency type—can contribute to higher costs, can cause additional losses and lower efficiency, can contribute to complexity of topology (or configuration), and can increase size and weight. Therefore, there is a need for transformer-less configurations for converting DC power to AC power. However, eliminating the transformer can cause other issues, such as, eliminating galvanic isolation between PV panels and the grid.
Although codes and standards for PV inverters (e.g., power converters for conversion of DC power from PV panels to AC power for an AC load) have begun to support such transformer-less configurations, there are several issues that need to be addressed. One problem is a safety hazard due to the lack of galvanic isolation between the PV panels and the grid. This can be addressed by requiring PV inverters to have extensive and reliable ground fault interruption mechanisms.
Another problem in transformer-less configurations is related to large, capacitive ground currents. Typical PV modules have significant capacitance (on the order of 100 nanofarads per kilowatt) between the PV cells that make up the module and the chassis of the PV panel. This is sometimes referred to as a parasitic capacitance. The chassis of a PV panel is typically required to be grounded, and therefore, the parasitic capacitance between the PV and the chassis also becomes a parasitic capacitance between PV and the ground. In conventional transformer isolated configurations, it is possible to connect either the positive or negative end of a PV string (e.g., a string of PV modules) to ground, thereby making the voltage across the parasitic capacitances zero or a constant direct current (DC) voltage. However, in conventional transformer-less configurations, such grounding can lead to short circuiting the PV voltage periodically as the inverter switches go through the pulse-width modulation (PWM) switching pattern. Therefore, such grounding is not typically used in conventional transformer-less configurations. As a consequence, during ungrounded operation the voltages across the parasitic capacitances between the PV cells and the ground, referred to as the common mode voltages, can pulsate between different values depending on the configuration (e.g., Vdc, 0, and −Vdc) at the switching frequency. This can result in large, undesirable capacitor currents that can interfere with normal operation.
Several variations of the conventional bridge type inverter topology have been proposed to mitigate the problems associated with the capacitive currents in transformer-less PV applications. These configurations typically involve the addition of switches and diodes to isolate the grid from the positive or negative terminals of the PV string during part of the switching cycle. This can reduce the common mode voltage, and thereby reduce the capacitive currents. However, it should be noted that these solutions can reduce the magnitude of pulsation in common mode voltage, but do not fully eliminate the pulsations.
Another problem in single-phase PV inverters, with or without transformers, is power pulsation at twice the line frequency (e.g., at 120 hertz (Hz) where the line frequency is 60 Hz, as in the United States). In order to capture maximum power from a PV string, the power drawn from the PV string should be kept as close to a constant power (e.g., a constant DC power) as possible. In contrast, power injected into a single-phase grid should have a constant component (e.g., a constant DC component) as well as a pulsating component AC component (e.g., 120 Hz AC). The difference between the DC power of the PV string and the combined power of the single-phase grid can be compensated for by utilizing a storage element. Conventionally, large electrolytic capacitors have been used to support such large power pulsations while keeping the DC link voltage fairly constant. However, electrolytic capacitors are among the leading causes of failure in PV inverters and represent a reliability concern. Furthermore, depending on the configuration, the 120 Hz power oscillation can be reflected in the power drawn from the PV panel resulting in reductions in efficiency that can substantially reduce energy harvest.
Accordingly, new circuits and methods for photoelectric inverters are desirable.